1. Field of the Invention
The present invention relates to ammonium polyphosphate solutions, and more particularly to techniques for impeding the settling of precipitate out of said solutions.
2. Description of the Prior Art
In liquid fertilizers containing ammonium polyphosphate, the nitrogen and the phosphate content can vary considerably. For example, a typical product of ammonium polyphosphate liquid contains 10% nitrogen and 34% P.sub.2 O.sub.5. Another possible grade of this product may be one containing 11% nitrogen and 37% P.sub.2 O.sub.5. There are numerous other grades which can be made by varying the ratio of nitrogen of P.sub.2 O.sub.5 as the product is manufactured or by addition of ammonia or water or other ingredients after the product is made. The P.sub.2 O.sub.5 present in liquid fertilizers containing ammonium polyphosphate is typically in the range of 40-75% poluphosphate with the remaining 60-25% present as orthophosphate. Further, the polyphosphate is present in various forms such as pyro-, tripoly-, and longer chain polymers.
In manufacturing the liquid ammonium polyphosphate fertilizer, it is often desirable to use wet process phosphoric acid as a source of phosphate. The primary reason is that wet process phosphoric acid is the cheapest source of phosphate. When wet process orthosphosphoric acid is used to make the ammonium polyphosphate fertilizer, the product will contain a number of impurities which were introduced with the wet process acid. Shortly after the ammoniation of the wet process phosphoric acid and subsequent condensation to ammonium polyphosphate, the impurities precipitate out of solution. It is know that these precipitates are chemicall complex salts of the forms: MgAl(NH.sub.4).sub.5 (P.sub.2 O.sub.7).sub.2 F.sub.2.6H.sub.2 O; Mg(NH.sub.4).sub.6 (P.sub.2 O.sub.7).sub.2.6H.sub.2 O; (NH.sub.4).sub.6 AlF.sub. 6 ; Mg(NH.sub.4).sub.2 P.sub.2 O.sub.7.4H.sub.2 O; Al(NH.sub.4).sub.2 P.sub.2 O.sub.7 OH.3H.sub.2 O, with the first of these being the predominant complex salt.
In one method of producing the liquid fertilizer, concentrated phosphoric acid is fed to a system where it is combined with anhydrous ammonia or other forms of ammonia. The wet process phosphoric acid is produced from the reaction of phosphate rock with sulfuric acid. A typical analysis of one type of wet process phosphoric acid is as follows:
______________________________________ % P.sub.2 O.sub.5 (Total) 60 - 62) % Polyphosphate 0 % MgO 0.85 % Fe.sub.2 O.sub.3 2.0 % Al.sub.2 O.sub.3 1.8 % F 0.7 ______________________________________
While the acid is clarified prior to concentration, the remaining impurities listed above create severe product quality problems.
After production of the liquid ammonium polyphosphate fertilizer, it is usually placed in storage for quite some time before it is applied to farm lands. It is seldom applied immediately after its production. It may remain in production storage, shipping vessels or customer storage tanks or systems for periods of several days to several months. During this period, complex material in the liquid precipitates and settles to the bottom of the container. Carbonized organics, typical of wet process phosporic acid, also settle out, compounding the problem. The organics, when mixed with other impurities settle to from a slimy, compact sludge layer. Often the entire liquid fertilizer will get to form a solid mass of material which cannot be readily removed from the storage containers. Sludge which forms in containers will compact to form a layer which is extremely difficult to remove. Settling in transportation vessels, storage tanks, and customer tanks results in product loss, handling problems, and high cost for cleaning tanks and equipment and general customer dissatisfaction.
This sludge material is also very difficult to apply to farm land. When the farmer or fertilizer applicator attempts to apply the liquid fertilizer to the soil, distributors or spray nozzles and equipment becomes plugged, requiring interruption of application to permit cleaning the pluggage out. Also, the pluggage results in uneven application to the soil of the liquid fertilizer and this results in extremely poor crop response because of poor utilization of the fertilizer nutrients. Therefore, solving the problem of the sludge formation becomes a major benefit.
In view of this serious precipitation problem, wet process orthophosphoric acid is not commonly used in the preparation of liquid ammonium polyphosphate fertilizers, or when it is used, it is used only in combination with economically unattractive procedures for eliminating the precipitate or for altering the nature of the precipitate so as to be less objectionable. For instance, it is known to attempt to solubilize the precipitate by the addition of fluoride ion to the solution prior to the crystal formation. One difficulty with this technique is the fact that the added fluorine renders the fertilizer toxic to animals, and hence, the fertilizer cannot be used as a feed supplement.
It is also known to try to suspend the crystals to prevent them from settling out. For instance, in Slack et al, U.S. Pat. Nos. 3,109,729 and 3,113,858, which refers to suspension type fertilizers, it is suggested to add clay to the solution which acts as a suspending agents. This technique, however, has not proven to be entirely satisfactory, since it increases the viscosity of fertilizer suspension and reduces the flowability and the clay settles to cause problems.
Preston, U.S. Pat. No. 3,183,073 , discloses the addition of sugar to prevent precipitation in high analysis liquid ammonium polyphosphate fertilizers. The addition of the sugar is disclosed to maintain the impurity precipitates which occur when the ammonium polyphosphate is derived from wet process phosphoric acid in suspension.
Miller, U.S. Pat. No. 3,022,153 suggests delaying the formation of a precipitate by additon of urea to the wet process phosphoric acid prior to ammoniation. This technique is disclosed to yield a liquid ammonium orthophosphate fertilizer solution which is stable for only 3-4 days before precipitation begins to occur. Miller indicates that if the fertilizer solution is to be held for longer than 3-4 days, it is necessary to add an amine after the completion of the ammoniation step. However, the addition of amine raises the pH of the fertilizer solution which can cause precipitation of the nutrients in the solution.
Burch et al, U.S. Pat. No. 3,625,672, discloses another technique for avoiding precipitation of magnesium impurities in liquid fertilizer solutions prepared by ammoniating wet process phosphoric acid. The technique of Burch et al is to ammoniate the wet process phosphoric acid until a pH of from 6.0 to 6.2 is achieved, then to add concentrated nitric acid to attain a pH from 5.0 to 5.6. In this manner, the tendency of the magnesium impurities to precipitate is reduced. However, it has been found that the addition of nitric acid alone only delays the formation of the objectionable precipitate, but does not prevent its formation.
Accordingly, a need continues to exist for a method which is capable of preventing the problems associated with precipitate formation in liquid fertilizers containing polyphosphate, which is prepared form wet process orthophosphoric acid or from wet process superphosphoric acid and hence is storage stable over extended periods of time without substantial precipitate settlement.